STEGANOGRAPHY, the art of hiding things in plain sight, is a trick as old as espionage.

Unlike its cousin, cryptography, which makes no attempt to disguise the existence of a message, but rather hides its meaning, a steganographic message need not be encyphered. What it does need to be is invisible—at least to those who are not the intended recipients. And that, in the modern world of the internet, is a crucial distinction. A censor can block a message he mistrusts, even if he cannot read it, thus putting the onus on the recipient to justify both the message and the fact it is encrypted. A well-crafted steganographic message, though, will never come to the censors’ attention in the first place. Which is the purpose of Collage, a system devised by Nick Feamster and his colleagues at the Georgia Institute of Technology.

Traditional steganography hides its message as, say, every 20th word in a letter, or as the colour of every hundredth pixel in an electronic image. Sophisticated analysis of such things might, though, notice something odd and thus flag a document for closer examination. Collage escapes notice by dividing the message into pieces, and then hiding these in electronic files posted to public websites, such as Flickr, Twitter and YouTube.

Scattering the message among multiple files and websites offers a number of advantages. For instance, the small amount of data in each file makes it difficult for a censor to notice anything odd unless all traffic on the network is subjected to advanced analysis techniques. Though possible in theory, the cost and effort of doing so makes this unfeasible in practice. More importantly, Collage’s design allows reconstruction of the original message even if only 60-80% of the files hiding it are recovered. Thus, even if a censor manages to block some of the files, users are still able to communicate.